* chore: migrate from base::StringPiece to std::string_view in keyboard_util.cc
* chore: migrate from base::StringPiece to std::string_view in error_thrower.cc
* chore: migrate from base::StringPiece to std::string_view in electron_api_web_contents.cc
* chore: migrate from base::StringPiece to std::string_view in gin_helper/dictionary.h
* chore: migrate from base::StringPiece to std::string_view in electron_api_url_loader.cc
* chore: phase out internal use of base:::StringPiece
`base::StringPiece` is being phased out upstream. Its code has been
removed upstream and it's just a typedef for `std::string_view`.
They haven't removed the typedef yet, so this PR tries to get ahead
of future breakage by migrating "internal" use (i.e. leaving alone the
places where the `base::StringPiece` name is coming from an upstream
method that we override).
Xref: https://bugs.chromium.org/p/chromium/issues/detail?id=691162
Xref: https://chromium-review.googlesource.com/c/chromium/src/+/4294483
Xref: https://docs.google.com/document/d/1d4RnD1uAE2t4iANR0nXy82ASIPGsPuw2mpO6v6T7JKs
* refactor: remove a few uses of native_mate/gfx_converter.h
* refactor: deprecate mate::EventEmitter
* refactor: add gin_helper::EventEmitter
* refactor: convert a few classes to use gin_helper::EventEmitter
* refactor: get rid of native_mate_converters/gfx_converter.h
* fix: follow native_mate on reporting errors
* fix: gin is weak at guessing parameter types
* fix: incorrect full class name
* fix: gin::Handle does not accept null
